How do I sterilise jars properly?

The oven method, why the lids are handled separately, and the detail that decides whether a preserve keeps for a year.

Difficulty: beginner
Time: 30 min
Read: 4 min
Safety: warning

Short answer

Wash jars in hot soapy water, rinse, and dry them in an oven at 140°C for fifteen minutes. Sterilise the lids separately in boiling water, since most have a rubber or plastic seal that oven heat damages. Fill while both jar and contents are hot.

Sterilising is not the same as washing. The point is to kill the organisms that would otherwise grow in the sugar and acid of a preserve over months in a cupboard, and the whole thing hinges on the jar being hot when it is filled.

Safety

Hot jars and hot preserve together are the main burn risk in a home kitchen — use a cloth or jar tongs, never wet hands, and never pour hot liquid into a cold jar or put a hot jar on a cold surface, as either will crack it. Sterilising deals with jars, not with the recipe: low-acid foods such as vegetables, meat and most sauces cannot be safely preserved by this method at home and need pressure canning or refrigeration.

Step by step

Check the jars before you start.Any chip on the rim means the lid cannot seal and the jar is not usable for preserving. Run a finger round every rim. Cracks anywhere mean the jar will fail under heat.
Wash thoroughly in hot soapy water.Including the threads at the top where residue collects. Rinse completely — detergent left behind taints the contents and can interfere with the seal.
Dry them in the oven.Upright on a baking tray, in an oven at around 140°C for fifteen minutes. Straight from washing, still wet, so they dry in the oven rather than on a cloth that reintroduces bacteria.
Do the lids separately in boiling water.Most lids have a rubber or plastic sealing compound that oven heat damages or distorts. Five minutes in boiling water, then lifted out with tongs onto clean kitchen paper.
Use the dishwasher as an alternative.A hot cycle with no detergent, timed so it finishes as the preserve is ready. Convenient, and the jars must be used straight from a hot machine rather than left to cool.
Time it so jars and preserve are ready together.This is the part that goes wrong. Hot into hot: hot jars, hot preserve. Cold preserve into hot jars, or hot preserve into cold jars, both crack glass.
Handle jars with a cloth or tongs.They come out of the oven genuinely hot. Never with wet hands or a damp cloth — the temperature shock cracks the glass and you are holding it at the time.
Fill to the right level.Leave the headspace the recipe states, usually around a centimetre. Too full and the seal fails; too empty and there is more air to spoil the contents.
Wipe the rim before sealing.Any preserve on the rim prevents a clean seal, and that is what lets mould in. A clean damp cloth around each rim before the lid goes on.
Seal immediately and tightly.Lid on straight away while everything is hot. As it cools, the contents contract and pull the lid down, which is what forms the vacuum seal.
Check the seal once cool.The centre of the lid should be concave and should not click when pressed. Anything that clicks has not sealed — refrigerate that jar and eat it within a few weeks.
Label with contents and date.Every jar, at the point of filling. Two similar preserves are indistinguishable in a cupboard three months later.

Tips

Do the jars while the fruit is cooking. The timing is the whole skill and it becomes automatic after one batch.
Keep jars from bought preserves — most are reusable, but buy new lids, since the sealing compound is not designed to be reused.
A jam funnel costs very little and eliminates almost all of the mess and the wiping of rims.

Common mistakes

Sterilising the lids in the oven with the jars — The sealing compound in most lids is rubber or plastic and distorts at oven temperature, so the jar never seals properly.
Filling cooled jars with hot preserve — The thermal shock cracks the glass, and a jar that does not crack has still lost the heat that helps form the seal. Hot into hot is the rule.

If it doesn't work

Lid clicks when pressed after cooling

Cause: Seal did not form — usually preserve on the rim or the lid not tightened while hot — Fix: That jar has not sealed. Refrigerate it and use within a few weeks rather than storing it in a cupboard.

Jar cracks when filled

Cause: Temperature difference between jar and contents — Fix: Both must be hot. Keep jars in the oven until the moment of filling.

Mould on the surface after a few months

Cause: Imperfect seal, or preserve left on the rim — Fix: Discard the jar — mould in a preserve is not dealt with by removing the top layer. Wipe rims carefully next time.

Jars come out of the oven with water marks

Cause: Rinsed in hard water — Fix: Cosmetic only and does not affect safety. A final rinse in filtered water avoids it.

Questions people ask

How do I sterilise jars in the oven?

Wash in hot soapy water, rinse, and put them wet and upright into an oven at around 140°C for fifteen minutes. Sterilise the lids separately in boiling water, since oven heat damages their seals.

Do I need to sterilise jars for jam?

Yes. Washing is not the same thing — sterilising kills what would otherwise grow over months in the cupboard. Fill while both the jars and the preserve are hot, which is what forms the seal.